In a stereo encoding technology, a left channel (L) signal and right channel (R) signal need to be down-mixed to obtain a monophonic (M) signal, and the M signal and sound field information of left and right channels that serves as a sideband signal are transmitted to a decoding end. The sound field information of the left and right channels includes a level difference between left and right channel signals and a phase difference between the left and right channel signals. The level difference between the left and right channel signals may be specifically ICLD (InterChannel Level Difference, interchannel level difference) or CLD (Channel Level Difference, channel level difference), and soon. The phase difference between the left and right channel signals may be specifically IPD (Interchannel Phase Difference, interchannel phase difference), and so on.
Current stereo signal down-mixing methods mainly include the following two:
Method 1: Use m(n)=0.5·(x1(n)+x2(n)) to obtain a monophonic signal m(n), where n indicates a time index, x1(n) and x2(n) indicate left and right channel time-domain signals respectively when the time index is n, and 0.5 indicates a down-mixing factor which may also be another value.
Method 2: Perform time-frequency conversion for the left and right channel signals, adjust the amplitudes and/or phases of the channel signals in a frequency domain, down-mix the channel signals, that have been adjusted, to obtain a frequency-domain monophonic signal, and convert the frequency-domain monophonic signal into a time-domain monophonic signal. Adjusting the phases of the channel signals means to use the phase of one channel signal as a benchmark to rotate the phase of another channel signal so that the phases of the two channel signals are the same.
During implementation of the present invention, the inventor finds that: in method 1, when the phases of the left and right channel signals are completely reverse and the amplitudes are the same, an obtained down-mixed signal is 0, and the decoding end fails to restore the left and right channel signals; in addition, when the phases of the left and right channel signals are not completely reverse, the obtained down-mixed signal may encounter energy loss. In method 2, if only the amplitudes of the signals in the frequency domain are adjusted, but the phases are not adjusted, the situation of 0 down-mixed signal and energy loss still occur; if the phases of the channel signals in the frequency domain are adjusted, when a benchmark channel signal is noise, it may occur that another signal is almost covered by the noise, and the phase of the down-mixed signal encounters a large jump when the phase of the benchmark channel signal changes greatly.